This source can be used in many ways according to the various values of the kinetic energy of the ions produced and can be used in ionic and microetching applications and more particularly in the equipment of accelerators of particles used in scientific and medical applications.
In ion sources with electronic cyclotronic resonance, the ions are obtained via the ionization inside a closed chamber, such as a hyperfrequency cavity, of a gaseous medium constituted by one or several metallic vapors or gases with the aid of electrons highly accelerated by means of electronic cyclotronic resonance. This resonance is obtained by virtue of the combined action of a high frequency (HF) electromagnetic field injected into the chamber containing the gas to be ionized, and a magnetic field existing in this chamber whose amplitude B satisfies the following electronic cyclotronic resonance condition: EQU B=f.2.pi.m/e,
in which e represents the charge of the electron, m its mass and f the frequency of the electromagnetic field.
In these sources, the quantity of ions able to be produced results from competition between two processes: firstly, the formation of ions via the electronic impact on neutral atoms constituting the gas to be ionized, and secondly the destruction of these same ions by recombining them singly or collectively when the latter collide with a neutral atom; this neutral atom may originate from the gas still not ionized or even be produced on the walls of the chamber via the impact of an ion on said walls.
This drawback can be avoided by confining the ions formed inside the chamber constituting the source, as well as the electrons used to ionize said ions. This is effected by creating inside the chamber radial and axial magnetic fields defining an "equimagnetic" surface having no contact with the walls of the chamber and concerning which the electronic cyclotronic resonance condition is satisfied. This surface has the shape of a rugby ball. The closer this equimagnetic surface is to the walls of the chamber, the more effective it is as it makes it possible to limit the presence volume of the neutral atoms and thus the quantity of ions/neutral atoms collisions. This surface also makes it possible to confine the ions and the electrons produced via ionization of the gas. By means of this confinement, the created electrons are able to bombard a given ion several times and to fully ionize it.
This type of source of ions has been described in the document filed on Mar. 13, 1986 in the name of the Applicant and published under the number FR-A-2 595 868.
FIG. 1 diagrammatically shows a source of ions according to the prior art. This source includes a chamber 1 constituting a resonant cavity able to be excited by a high frequency (HF) electromagnetic field. This electromagnetic field is produced by an electromagnetic wave generator 3; it is introduced inside the chamber 1 by means of a wave guide 5 and a transition cavity 20.
This source also includes a magnetic structure (7, 9, 11) externally shielded whose armouring 11 makes it possible to only magnetize the volume useful for electronic cyclotronic resonance in the chamber 1.
Apart from the armouring 11, this magnetic structure includes permanent magnets 7 and solenoids 9 disposed around the chamber 1 and respectively creating one radial magnetic field and one axial magnetic field. These two magnetic fields are superimposed and distributed inside the entire chamber ; they thus form one resultant magnetic field which defines a resonant equimagnetic surface 13 inside the chamber 1.
A magnetic axis 15, which is also the longitudinal axis of the source, traverses the armouring 11 through two openings 17 and 19 disposed inside said armouring 11 so as to respectively allow for the extraction of the ions from the chamber 1, as well as the introduction of electromagnetic waves and gaseous or solid samples.
One first and one second dielectric pipes 23 and 21 connect the opening 19 of the armouring 11 to respective openings 25 and 27 of the transition cavity 20, these openings being situated on the lateral faces of the cavity 20 which has the shape of a cube. The relation of the diameters of these two pipes 21, 23 is such that it is possible to assimilate them with a characteristic coaxial impedance line of about 85 .OMEGA.. This coaxial line preferably propagates an Electromagnetic Transversal electromagnetic mode (ETM) in which the electromagnetic field E is transversal to the propagation direction of the waves and perpendicular to the surface of the conductors, that is the pipes 21, 23.
In order to ionize a gas, said gas is introduced into the chamber 1 by means of a gas pipe 30 connected to the opening 27 of the transition cavity 20. The gas and the electromagnetic waves introduced into the cavity 20 are transmitted to the chamber 1 by the first and second pipes 21 and 23, the role of the first pipe being to transmit said waves towards said chamber and to inject them there along the longitudinal axis 15.
Inside the chamber 1, the association of the axial magnetic field with the electromagnetic field makes it possible to fully ionize the gas introduced. The electrons produced are then highly accelerated by means of electronic cyclotronic resonance, which results in the formation of a plasma of hot electrons confined inside the volume limited by the equimagnetic surface 13.
The ions thus formed inside the chamber 1 are extracted from the latter by means of an extraction electric field generated by a potential difference applied between an electrode 31 and the chamber 1. The electrode 31 and the chamber are both connected to an electric power feed source 33, the electrode 31 being positioned outside the opening 17 of the chamber 1.
So as to control the intensity of the ion current, it is possible to control the average power of the electromagnetic field by acting on a pulse generator 35 situated upstream of a power feed source 37 connected to the electromagnetic wave generator. Said pulse generator 35 controls said power source 37 by adjusting the effective cycle, namely the ratio between the duration of one pulse and the period of the pulses.
Furthermore, full pressure measuring means 39 are connected to one input of a comparator 41 whose output is connected to a valve 43 of the gas pipe 30. On one second input of the comparator 41, a reference voltage R is applied and compared with the measured value of the ions current so as to provide at the comparator outlet the value to be transmitted to the valve 43. This valve 43 is able to act on the quantity of gas to be introduced into the chamber 1 so as to automatically adjust the ions current.
In addition, an adaptation piston 45 connected to a third lateral opening 29 of the cavity 20 makes it possible to adjust the internal volume of said cavity 20. The adjustment of said piston 45 is used so as to tune all the internal volumes of the cavity 20 to the frequency of the electromagnetic waves so as to obtain a minimum number of reflected waves, that is waves which return to the wave generator 3. When these internal volumes are tuned to the frequency of the electromagnetic waves, the waves injected into the cavity 20 by the generator 3 are virtually fully transmitted by the pipes 21 and 23 to the chamber 1 containing the plasma and are then absorbed by the equimagnetic surface 13.
In this source of ions of the prior art, the second pipe 23 is transparent to the electromagnetic waves at its extremity 23a, this extremity being adjacent to the opening 19 of the chamber 1 situated opposite the armouring 11.
Inside the internal volume of this transparent portion 23a, there is an axial magnetic field originating from solenoids, one electromagnetic field and one high gas pressure. The electromagnetic field derives from electromagnetic waves transmitted between the first pipe 21 and one non-transparent portion 23b of the second pipe 23 and which traverse the transparent portion 23a of the second pipe 23. Owing to this, electronic cyclotronic resonance may take place inside the extremity 23a of the second pipe 23 inside a volume where there exists high gas pressure. The denser is the plasma produced by electronic cyclotronic resonance inside the extremity 23a, the better is transmission of the electromagnetic waves, this dense plasma cord becoming a conductor. Furthermore, this plasma cord has the same outer diameter as the portion 23b of the second pipe. The characteristic impedance of the coaxial line is thus not modified, which makes it possible to avoid reflection of the electromagnetic waves.
This extremity transparent to the electromagnetic waves thus constitutes a self-adjusted pre-ionization stage where the excess incident power of the electromagnetic waves is transmitted without reflection as far as the electronic cyclotronic resonance zone constituted by the equimagnetic surface 13.
Therefore, so as to optimize a source of ions as described in the prior art, it is firstly necessary to adjust the volume of the transition cavity 20 by acting on the adaptation piston 45 and secondly adjust the intensity of the current in the solenoids 9. These adjustments, even if carried out by an experienced operator, may turn out to be extremely long: they may last hours, indeed days without nevertheless resulting in obtaining the performance optimum of the source. In fact, these adjustments do not obey any known rule used to optimize the source of ions.